It is possible that the bubbles are either propellant from the paint or some sort of contamination of the surface. If you handle the surface with your fingers or dirty gloves, that contamination may have transferred to the plastic. Try using mist or light coats of paint until you acheive the paint depth you desire.

As mentioned above and by personal experience, it's definitely from spraying too close on wet coats; the propellant that is normally dissipated before hitting the surface gets trapped under the surface by the surplus of wet paint from a too-close, too-wet coat.

Zoom Zoom, aka Bob Downie
My Fotki Album
The only cure for modeler's ADD is "final assembly and decal placement"

I have also experienced this problem, but only with Tamiya paints, and at least for me, only with spraying the final wet coats with TS-13 Clear.

Bob's explanation is right on the money.

Back off the spray can another couple of inches and you will be fine.

However, the only way to get rid of all the bubbles is to remove the paint entirely from the damaged area and start over. Otherwise you run the risk of the bubbled surface, no matter who much you have sanded it smooth, bubbling through to the new top coat.